Development of Trans-1,4-Polyisoprene Shape-Memory Polymer Composites Reinforced with Carbon Nanotubes Modified by Polydopamine

In this study, which was inspired by mussel-biomimetic bonding research, carbon nanotubes (CNTs) were interfacially modified with polydopamine (PDA) to prepare a novel nano-filler (CNTs@PDA). The structure and properties of the CNTs@PDA were studied using scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and thermogravimetric analysis (TGA). The CNTs and the CNTs@PDA were used as nanofillers and melt-blended into trans-1,4 polyisoprene (TPI) to create shape-memory polymer composites. The thermal stability, mechanical properties, and shape-memory properties of the TPI/CNTs and TPI/CNTs@PDA composites were systematically studied. The results demonstrate that these modifications enhanced the interfacial interaction, thermal stability, and mechanical properties of TPI/CNTs@PDA composites while maintaining shape-memory performance.


Introduction
Shape-memory polymer (SMP) refers to a type of smart polymer that can return to its initial shape from a temporary deformed shape under external stimuli such as heat, light, electricity, magnetism, pH value, metal ions, and many other physical and chemical stimuli [1][2][3]. Because of their unique shape-recovery function, SMPs have broad application prospects in many fields, including biological medical devices, aerospace engineering, textiles, and anti-counterfeiting [4][5][6]. Thermally induced SMP is an important branch of shape-memory polymer stimulation that has been widely studied and has many industrial applications [7][8][9]. As an important component of thermally induced SMPs, trans-1,4-polyisoprene (TPI) and its composites have been used in biomedical and protective equipment applications due to their fast deformation speed, high recovery rate, and high recovery accuracy [10]. However, poor thermal stability and weak mechanical properties limit the potential of TPI shape-memory polymers.
In recent years, researchers have found that the shape-memory and mechanical properties of shape-memory polymers can be effectively enhanced by integrating nano-fillers into a polymer's matrix [11][12][13]. Carbon nanotubes (CNTs) are a new type of one-dimensional nanomaterial where the carbon atoms in the tube walls form a hexagonal network structure due to sp 2 hybridization; the basic network is the same as that of graphene. The C=C covalent bond formed by sp 2 hybridization is one of the strongest covalent bonds in nature, so carbon nanotubes have extremely high mechanical properties [14,15]. The The prepared CNTs@PDA nano-filler was melt-blended with TPI in a 70 °C for 15 min. TPI composites were prepared by adding 2.0 phr (part rubber) SA, 4.0 phr ZnO, 2.0 phr 4010NA, 1.2 phr NOBS, and 1.5 phr sul CNTs@PDA (0 phr, 0.6 phr, 1.2 phr, 1.8 phr, 2.4 phr, and 3.0 phr, respectiv matrix. After standing at room temperature for 24 h, the composite was h vacuum at 5 MPa and 150 °C for 10 min to form 2 mm thick sheets. In additi group was created according to the same method yielding 0.6 phr TPI/CN material that contained exclusively CNTs.

Characterizations
Fourier transform infrared (FTIR) spectra were recorded using a Four ter (Nicolet MAGNA-IR560) to characterize the chemical structures CNTs@PDA.
Scanning electron microscopy (SEM) was used to analyze the microst nanocomposite samples. The SEM micrographs were obtained by JSM-6 Jeol).
Raman spectra were obtained using a multichannel confocal spectrom Scientific LabRAM HR Evolution) with a laser wavelength of 535 nm.
Thermogravimetric analysis (TGA) was performed on a STA449C ( tzsch). Under a nitrogen environment, the range of test temperatures was °C at a heating rate of 10 °C/min. Dynamic mechanical analysis (DMA) was carried out by using a DMA can TA). The frequency and heating rate were set at 1 Hz and 3 °C/min from Differential scanning calorimetry (DSC) analysis was used by a DSCQ TA) under a nitrogen atmosphere. Samples with a mass of 8-10 mg were The prepared CNTs@PDA nano-filler was melt-blended with TPI in an open mill at 70 • C for 15 min. TPI composites were prepared by adding 2.0 phr (parts per hundred rubber) SA, 4.0 phr ZnO, 2.0 phr 4010NA, 1.2 phr NOBS, and 1.5 phr sulfur along with CNTs@PDA (0 phr, 0.6 phr, 1.2 phr, 1.8 phr, 2.4 phr, and 3.0 phr, respectively) to the TPI matrix. After standing at room temperature for 24 h, the composite was hot pressed in a vacuum at 5 MPa and 150 • C for 10 min to form 2 mm thick sheets. In addition, the control group was created according to the same method yielding 0.6 phr TPI/CNTs composite material that contained exclusively CNTs.

Characterizations
Fourier transform infrared (FTIR) spectra were recorded using a Fourier spectrometer (Nicolet MAGNA-IR560) to characterize the chemical structures of CNTs and CNTs@PDA.
Scanning electron microscopy (SEM) was used to analyze the microstructures of the nanocomposite samples. The SEM micrographs were obtained by JSM-6360LV (Japan Jeol).
Raman spectra were obtained using a multichannel confocal spectrometer (HORIBA Scientific LabRAM HR Evolution) with a laser wavelength of 535 nm.
X-ray diffraction (XRD) analysis was recorded by D & Advance (Germany Bruker) with CuK α radiation (λ = 0.154 nm), and the diffraction angle was decreased from 5 • to 80 • with a scanning rate of 3 • /min. X-ray photoelectron spectroscopy (XPS) was performed by an X-ray photoelectron spectrometer (KRATOS, Axis UltraDLD).
Thermogravimetric analysis (TGA) was performed on a STA449C (Germany Netzsch). Under a nitrogen environment, the range of test temperatures was from 40 to 800 • C at a heating rate of 10 • C/min. Dynamic mechanical analysis (DMA) was carried out by using a DMAQ800 (American TA). The frequency and heating rate were set at 1 Hz and 3 • C/min from −75 to 50 • C.
Differential scanning calorimetry (DSC) analysis was used by a DSCQ200 (American TA) under a nitrogen atmosphere. Samples with a mass of 8-10 mg were maintained at 100 • C for 5 min then cooled to −80 • C at 10 • C/min for 5 min, and samples were heated to 100 • C at 10 • C/min. The degree of crystallinity (X c ) for each portion of the sample was calculated by the following equation: where ∆H m and ∆H m * are the melting enthalpy of a certain polymer portion and that of pure polymer (ca. 186.8 J/g for TPI), respectively [38]. The shape-memory properties of the composites were analyzed by DMA. Firstly, the samples were equilibrated at 60 • C and then stretched at a stress of 0.4 MPa. Secondly, the sample was maintained at a constant stress and cooled down to −10 • C quickly, equilibrated at this temperature for 5 min, and then the stress was removed. In the last step, the samples were heated to 60 • C and equilibrated. The fixing ratio (R f ) and recovery ratio (R r ) were used to calculate the following formulas. Figure 2 shows the FTIR spectra of CNTs and CNTs@PDA samples. For the spectra of the original CNTs, obvious bands were observed at 3436 and 1574 cm −1 , which may have been caused by the adsorption of -OH groups on CNTs and the C=C stretching vibration of CNTs while at 1720 cm −1 , and the corresponding stretching vibration peak of C=O was observed. The absorption peaks at 2924 and 2855 cm −1 corresponded to the stretching vibration of aliphatic C-H, and the intensity of the absorption peaks of CNTs@PDA increases significantly, which is caused by the increase in the surface carbon content of CNTs after coating. In addition, after the auto-oxidation polymerization of dopamine, PDA was formed, which had a strong absorption peak at 1260 and 1620 cm −1 , which was caused by the stretching vibration of C-O and the aromatic ring, respectively. There was a wide peak at 3445 cm −1 , which was caused by the combination of the two characteristic peaks resulting from the abundant N-H and -OH on the PDA. These results clearly indicate that the CNTs were successfully functionalized by the PDA. pure polymer (ca. 186.8 J/g for TPI), respectively [38].

Characterization of CNTs and CNTs@PDA
The shape-memory properties of the composites were analyzed by DM samples were equilibrated at 60 °C and then stretched at a stress of 0.4 MPa. sample was maintained at a constant stress and cooled down to −10 °C q brated at this temperature for 5 min, and then the stress was removed. In th samples were heated to 60 °C and equilibrated. The fixing ratio (Rf) and reco were used to calculate the following formulas.  Figure 2 shows the FTIR spectra of CNTs and CNTs@PDA samples. F of the original CNTs, obvious bands were observed at 3436 and 1574 cm − have been caused by the adsorption of -OH groups on CNTs and the C=C bration of CNTs while at 1720 cm −1 , and the corresponding stretching vibr C=O was observed. The absorption peaks at 2924 and 2855 cm −1 corresp stretching vibration of aliphatic C-H, and the intensity of the absorpt CNTs@PDA increases significantly, which is caused by the increase in the s content of CNTs after coating. In addition, after the auto-oxidation polymer pamine, PDA was formed, which had a strong absorption peak at 1260 a which was caused by the stretching vibration of C-O and the aromatic ring There was a wide peak at 3445 cm −1 , which was caused by the combinati characteristic peaks resulting from the abundant N-H and -OH on the PDA. clearly indicate that the CNTs were successfully functionalized by the PDA In order to explore the morphological changes of CNTs before and after the surface morphology of CNTs was characterized by SEM. The SEM im and CNTs@PDA are shown in Figure 3. As can be seen from the figure, unmodified CNTs was relatively smooth and clear in outline, while the  In order to explore the morphological changes of CNTs before and after modification, the surface morphology of CNTs was characterized by SEM. The SEM images of CNTs and CNTs@PDA are shown in Figure 3. As can be seen from the figure, the surface of unmodified CNTs was relatively smooth and clear in outline, while the surface of the CNTs@PDA had an obvious coating layer that was dense and complete in structure, and the diameter of the tube was increased relative to that of the CNTs. The unmodified CNTs also had a lot of entanglement and agglomeration. Comparatively, the dispersibility of the CNTs@PDA was significantly improved due to the PDA coating and its large number of active surface groups. In addition, the PDA coating formed a physical steric hindrance between the CNTs, which effectively prevented the agglomeration of the CNTs. Therefore, we determined that the PDA had both successfully coated the CNTs and provided additional benefits.

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CNTs@PDA had an obvious coating layer that was dense and complete in structure, and the diameter of the tube was increased relative to that of the CNTs. The unmodified CNTs also had a lot of entanglement and agglomeration. Comparatively, the dispersibility of the CNTs@PDA was significantly improved due to the PDA coating and its large number of active surface groups. In addition, the PDA coating formed a physical steric hindrance between the CNTs, which effectively prevented the agglomeration of the CNTs. Therefore, we determined that the PDA had both successfully coated the CNTs and provided additional benefits. Raman spectroscopy can distinguish between the hybrid forms of carbon atoms and is also an effective method to examine the structural characteristics of CNTs. The Raman spectra of the CNTs and the CNTs@PDA were both double peaks, as shown in Figure 4. The Raman spectra of CNTs showed a D band (1334 cm −1 ) and a G band (1581 cm −1 ), indicating that the graphitic structure was preserved in CNTs and CNTs@PDA [39]. If R is defined as the intensity ratio of peak D to peak G, the value of R can reflect the functionalization degree and the integrity of the CNTs graphite structure [40]. After calculation, the R values of the CNTs and the CNTs@PDA were 1.27 and 1.33, respectively. In contrast, the R value of the CNTs@PDA was only slightly greater than that of the CNTs, which was due to the introduction of extra heteroatoms (N), resulting in the decomposition of the graphitic structure, producing more defects and disordered areas within CNTs [41]. Raman spectroscopy can distinguish between the hybrid forms of carbon atoms and is also an effective method to examine the structural characteristics of CNTs. The Raman spectra of the CNTs and the CNTs@PDA were both double peaks, as shown in Figure 4. The Raman spectra of CNTs showed a D band (1334 cm −1 ) and a G band (1581 cm −1 ), indicating that the graphitic structure was preserved in CNTs and CNTs@PDA [39]. If R is defined as the intensity ratio of peak D to peak G, the value of R can reflect the functionalization degree and the integrity of the CNTs graphite structure [40]. After calculation, the R values of the CNTs and the CNTs@PDA were 1.27 and 1.33, respectively. In contrast, the R value of the CNTs@PDA was only slightly greater than that of the CNTs, which was due to the introduction of extra heteroatoms (N), resulting in the decomposition of the graphitic structure, producing more defects and disordered areas within CNTs [41].
XPS is an effective means of detecting the surface composition, chemical state, and chemical bond information of materials. The XPS curves and related data are shown in Figure 5 and Table 1, respectively. Figure 5a,b shows the full XPS spectrum of the CNTs and the CNTs@PDA, in which the CNTs and the CNTs@PDA could be seen in the two groups of main peaks corresponding to the C1s and the O1s near 284 and 533 eV. In addition to the C1s peak of 284 eV and the O1s peak of 533 eV in the full spectrum of the CNTs@PDA, the N1s peak also appeared near 400 eV, which confirmed the existence of the PDA. defined as the intensity ratio of peak D to peak G, the value of R can reflect the fu alization degree and the integrity of the CNTs graphite structure [40]. After calc the R values of the CNTs and the CNTs@PDA were 1.27 and 1.33, respectively. In c the R value of the CNTs@PDA was only slightly greater than that of the CNTs, wh due to the introduction of extra heteroatoms (N), resulting in the decomposition graphitic structure, producing more defects and disordered areas within CNTs [4 XPS is an effective means of detecting the surface composition, chemical sta chemical bond information of materials. The XPS curves and related data are sh Figure 5 and Table 1, respectively. Figure 5a,b shows the full XPS spectrum of th and the CNTs@PDA, in which the CNTs and the CNTs@PDA could be seen in t     The thermal stability of the CNTs@PDA was characterized by thermogravimetric analyzer. Figure 6 shows the thermogravimetric curves of the CNTs and the CNTs@PDA. As shown in the figure, the unmodified CNTs did not show significant weight loss during the whole thermal analysis process, which was in line with the excellent heat resistance characteristics of carbon nanomaterials. While the CNTs@PDA had begun to show weight loss at 250 • C, they had an obvious weight loss between 300 and 600 • C, which was mainly due to the thermal decomposition of the polydopamine. When the temperature increased to 800 • C, the thermal weight loss of the CNTs@PDA accounted for about 19.3% of the total weight, showing that the PDA had been successfully coated on the surface of the CNTs. The thermal stability of the CNTs@PDA was characterized by thermogravimetric analyzer. Figure 6 shows the thermogravimetric curves of the CNTs and the CNTs@PDA. As shown in the figure, the unmodified CNTs did not show significant weight loss during the whole thermal analysis process, which was in line with the excellent heat resistance characteristics of carbon nanomaterials. While the CNTs@PDA had begun to show weight loss at 250 °C, they had an obvious weight loss between 300 and 600 °C, which was mainly due to the thermal decomposition of the polydopamine. When the temperature increased to 800 °C, the thermal weight loss of the CNTs@PDA accounted for about 19.3% of the total weight, showing that the PDA had been successfully coated on the surface of the CNTs.

Crystallization Properties
The crystallization and crosslinking structure of TPI plays an important role in its mechanical and shape-memory properties. We used DSC to characterize the crystallization and melt-transition characteristics of TPI composites. The DSC curves and related data are shown in Figure 7 and Table 2, respectively. The results show that the melting temperature (Tm), crystallization temperature (Tc), melting enthalpy (∆Hm), and crystallinity (Xc) of the TPI/CNTs@PDA composites were higher than those of neat TPI and TPI/CNTs. The reason was that the agglomeration of neat CNTs in the matrix hindered the crystallization of the TPI molecular chains and led to the decrease in the crystallinity of the materials [42]. The results also indicate that the CNTs@PDA had better dispersion in the TPI composites. It is worth noting that the crystallinity of the composites first increased and then decreased with the additional amount of the CNTs@PDA. Two possibilities could explain this phenomenon: On the one hand, the CNTs@PDA may have acted as a nucleating agent in the composites, which could have promoted the crystallization process of the chain segment. On the other hand, when the total amount of the CNTs@PDA reached a certain temperature, the strong interaction between the CNTs and the polymer greatly reduced the regularity of the TPI molecular chain, which was not conducive to its crystallization, resulting in a decrease in the TPI crystallinity.

Crystallization Properties
The crystallization and crosslinking structure of TPI plays an important role in its mechanical and shape-memory properties. We used DSC to characterize the crystallization and melt-transition characteristics of TPI composites. The DSC curves and related data are shown in Figure 7 and Table 2, respectively. The results show that the melting temperature (T m ), crystallization temperature (T c ), melting enthalpy (∆H m ), and crystallinity (X c ) of the TPI/CNTs@PDA composites were higher than those of neat TPI and TPI/CNTs. The reason was that the agglomeration of neat CNTs in the matrix hindered the crystallization of the TPI molecular chains and led to the decrease in the crystallinity of the materials [42]. The results also indicate that the CNTs@PDA had better dispersion in the TPI composites. It is worth noting that the crystallinity of the composites first increased and then decreased with the additional amount of the CNTs@PDA. Two possibilities could explain this phenomenon: On the one hand, the CNTs@PDA may have acted as a nucleating agent in the composites, which could have promoted the crystallization process of the chain segment. On the other hand, when the total amount of the CNTs@PDA reached a certain temperature, the strong interaction between the CNTs and the polymer greatly reduced the regularity of the TPI molecular chain, which was not conducive to its crystallization, resulting in a decrease in the TPI crystallinity.
ities could explain this phenomenon: On the one hand, the CNTs@PDA may have acted as a nucleating agent in the composites, which could have promoted the crystallization process of the chain segment. On the other hand, when the total amount of the CNTs@PDA reached a certain temperature, the strong interaction between the CNTs and the polymer greatly reduced the regularity of the TPI molecular chain, which was not conducive to its crystallization, resulting in a decrease in the TPI crystallinity.  The TPI was a partially crystalline polymer material, which had two crystal forms: α and β. As shown in the XRD pattern in Figure 8, there were obvious characteristic peaks in the TPI composite material, of which the XRD diffraction angles (2θ) of the α and β crystal forms were 17.9 (α), 26.7 (α), 18.7 (β), and 22.7 (β) [43]. The three peaks between 30 • and 40 • corresponded to the characteristic peaks of ZnO [44]. In addition, there were no new crystallization peaks in the XRD curves of all the samples, which indicated that the crystal structure of the TPI composites had not been destroyed.  The TPI was a partially crystalline polymer material, which had two cr and β. As shown in the XRD pattern in Figure 8, there were obvious charac in the TPI composite material, of which the XRD diffraction angles (2θ) of th tal forms were 17.9 (α), 26.7 (α), 18.7 (β), and 22.7 (β) [43]. The three peaks bet 40° corresponded to the characteristic peaks of ZnO [44]. In addition, there crystallization peaks in the XRD curves of all the samples, which indicated th structure of the TPI composites had not been destroyed.

Thermal Properties
The TGA and DTG curves of the sample are shown in Figure 9, while t

Thermal Properties
The TGA and DTG curves of the sample are shown in Figure 9, while the main characteristic parameters, such as the initial degradation temperature (T 5% ), the maximum thermal degradation temperature (T max ) and carbon residue at 600 • C, are summarized in Table 3. Under a nitrogen atmosphere, the TPI/CNTs@PDA system had a higher T 5% , T max , and carbon residue than the neat TPI and TPI/CNTs systems. With the increase in CNTs@PDA content, T 5% , T max and carbon residue of TPI composites were increased by varying degrees. This could be attributed to the fact that polydopamine has a large number of benzene rings and hydroxyl structures. A large number of hydroxyl groups means crystalline water is formed more easily, and since the polymer loses water during combustion, it reduces the temperature as well. At the same time, a rich benzene ring structure can form carbon with high thermal stability that can then accumulate on the surface of the TPI matrix and form a dense carbon layer. As a thermal insulation layer, the thicker carbon layer may not only slow down the process of thermal degradation but may also prevent heat transfer to the interior of the remaining composite materials. The complementary interaction of the dense carbon layer structure and thermal insulation barrier yielded excellent thermal stability, and they acted upon the TPI/CNTs@PDA composites. The DTG curve showed that the CNTs@PDA not only promoted the formation of carbon but also slowed down the mass loss rate of the TPI composites during their degradation at high temperatures. The further decomposition of the matrix was delayed, and the carbon yield was increased, which had a positive effect on the thermal stability of the composites.

Mechanical Properties
The tensile strength properties of TPI composites at room temperature are shown in Figure 10. The tensile strength, elongation-at-break and Young's modulus of neat TPI were 17.7 MPa, 380.5% and 0.052 GPa, respectively. The tensile strength , elongation-atbreak and Young's modulus of the composites were increased to 18.8 MPa, 387.6% and 0.055 GPa, respectively, by adding 0.6 phr of the CNTs, which was mainly due to the unique mechanical and physical properties of CNTs, as well as their high specific surface area and aspect ratio [45,46]. When the same amount of CNTs@PDA (0.6 phr) was added, the tensile strength, elongation-at-break and Young's modulus of the composites were 19.8 MPa, 416.9% and 0.06 GPa, respectively, which showed a better strengthening effect, as compared to the CNTs alone. This was due to the fact that the van der Waals force interaction between the CNTs was relatively strong and was easy to agglomerate, which made the distribution of the CNTs in the TPI matrix uneven, and the agglomerates formed  The DTG curve showed that the CNTs@PDA not only promoted the formation of carbon but also slowed down the mass loss rate of the TPI composites during their degradation at high temperatures. The further decomposition of the matrix was delayed, and the carbon yield was increased, which had a positive effect on the thermal stability of the composites.

Mechanical Properties
The tensile strength properties of TPI composites at room temperature are shown in Figure 10. The tensile strength, elongation-at-break and Young's modulus of neat TPI were 17.7 MPa, 380.5% and 0.052 GPa, respectively. The tensile strength, elongation-at-break and Young's modulus of the composites were increased to 18.8 MPa, 387.6% and 0.055 GPa, respectively, by adding 0.6 phr of the CNTs, which was mainly due to the unique mechanical and physical properties of CNTs, as well as their high specific surface area and aspect ratio [45,46]. When the same amount of CNTs@PDA (0.6 phr) was added, the tensile strength, elongation-at-break and Young's modulus of the composites were 19.8 MPa, 416.9% and 0.06 GPa, respectively, which showed a better strengthening effect, as compared to the CNTs alone. This was due to the fact that the van der Waals force interaction between the CNTs was relatively strong and was easy to agglomerate, which made the distribution of the CNTs in the TPI matrix uneven, and the agglomerates formed small defects in the matrix, which reduced the tensile properties of the composites. The dispersion of the CNTs@PDA in the TPI was better, which could effectively avoid the agglomeration phenomenon, so the tensile properties of the TPI composites may prove beneficial. In addition, the tensile strength, elongation-at-break and Young's modulus of the composite reached the highest values when the CNTs@PDA content was 2.4 phr, which were 31.64%, 13.07% and 0.066 GPa higher, respectively, than that of the neat TPI. The tensile properties of the TPI composites improved with the addition of the CNTs@PDA under high load, which was a result of the CNTs@PDA thorough dispersion in the TPI and the strong interfacial bonding between the CNTs and the TPI matrix.  The DMA curves of the storage modulus (E′) and the loss factor (tan δ) versus the temperature of the samples are shown in Figure 11. Below the glass transition temperature (Tg), the addition of CNTs increased the storage modulus (E') of the TPI composite, as shown in Figure 11a. This result is associated with the hindered movement of the crosslinked molecular chain when CNTs were added into the TPI matrix. In addition, due to the high rigidity of the CNTs, they acted as a skeleton, which led to the improvement of the rigidity of the composites and was reflected by the increased energy storage modulus. As compared to the TPI/CNTs, the E' of the TPI/CNTs@PDA composites increased when the same content of the CNTs@PDA had been added. This was due to the abundant amounts of amine and catechol groups in the PDA coating layer, which improved the interface interaction between the CNTs@PDA surface and the TPI matrix [47]. However, when the amount of CNTs@PDA exceeded 2.4 phr, the E' of the composites decreased due to the abundant catechol groups, which not only promoted the dispersions of the CNTs in the TPI matrix but also enabled hydrogen bonding between the PDA coating layers. This resulted in a bridge between the CNTs and, therefore, their agglomeration and resultant defects. These defects formed stress concentration points in the matrix, leading to a decrease in E'. Figure 11b shows that when CNTs@PDA was added to the composites, the Tg was −48.5 °C, a reduction of 1.2 °C, as compared to the neat TPI. Tg gradually decreased and trended downward when the element content was increased, which in turn increased the distance between the molecular chains and promoted their movement. In addition, with the continuous increase in CNTs@PDA content, the loss factor of the samples tended to decrease, and the loss factor of the samples were lower than that of the neat TPI. The DMA curves of the storage modulus (E ) and the loss factor (tan δ) versus the temperature of the samples are shown in Figure 11. Below the glass transition temperature (T g ), the addition of CNTs increased the storage modulus (E') of the TPI composite, as shown in Figure 11a. This result is associated with the hindered movement of the crosslinked molecular chain when CNTs were added into the TPI matrix. In addition, due to the high rigidity of the CNTs, they acted as a skeleton, which led to the improvement of the rigidity of the composites and was reflected by the increased energy storage modulus. As compared to the TPI/CNTs, the E' of the TPI/CNTs@PDA composites increased when the same content of the CNTs@PDA had been added. This was due to the abundant amounts of amine and catechol groups in the PDA coating layer, which improved the interface interaction between the CNTs@PDA surface and the TPI matrix [47]. However, when the amount of CNTs@PDA exceeded 2.4 phr, the E' of the composites decreased due to the abundant catechol groups, which not only promoted the dispersions of the CNTs in the TPI matrix but also enabled hydrogen bonding between the PDA coating layers. This resulted in a bridge between the CNTs and, therefore, their agglomeration and resultant defects. These defects formed stress concentration points in the matrix, leading to a decrease in E'. Figure 11b shows that when CNTs@PDA was added to the composites, the T g was −48.5 • C, a reduction of 1.2 • C, as compared to the neat TPI. T g gradually decreased and trended downward when the element content was increased, which in turn increased the distance between the molecular chains and promoted their movement. In addition, with the continuous increase in CNTs@PDA content, the loss factor of the samples tended to decrease, and the loss factor of the samples were lower than that of the neat TPI. The dispersion state of the nano-fillers in the composites had an impact on their mechanical properties. To further study the dispersion and aggregation of the CNTs@PDA in the TPI, the sectional morphology of the composite was investigated with SEM. The cross-section of the neat TPI was relatively flat and smooth with typical brittle fracture characteristics. Figure 12b shows that the CNTs had a large-scale aggregation, and the cross-section became coarser, as compared to that of the neat TPI. Even though they had the same additional contents, the cross-section of the TPI/CNTs@PDA composites had no aggregation, indicating that the PDA further improved the dispersion of the CNTs in the TPI matrix. In addition, with the gradually increased CNTs@PDA content, a large number of small cracks appeared in the composite, and the density and depth gradually increased. This fracture behavior was due to the strong interfacial interaction between the PDA-modified CNTs and the TPI matrix. More effective stress was transferred from the TPI matrix to the CNTs with high modulus, which inhibited the crack propagation in the matrix on a larger scale, leading to the roughness seen in the cross-section of the composites. The addition of the PDA-coated CNTs improved the fracture toughness of the TPI matrix. However, when the content of CNTs@PDA exceeded 2.4 phr, there was still some small-scale aggregation, and the crack density decreased to a certain extent, as shown in Figure 12g. The suggested cause for this could be that with the increased packing content, the CNTs coated with PDA could still have had a strong association with each other, which could have led to their weak dispersion in the TPI matrix. The dispersion state of the nano-fillers in the composites had an impact on their mechanical properties. To further study the dispersion and aggregation of the CNTs@PDA in the TPI, the sectional morphology of the composite was investigated with SEM. The cross-section of the neat TPI was relatively flat and smooth with typical brittle fracture characteristics. Figure 12b shows that the CNTs had a large-scale aggregation, and the cross-section became coarser, as compared to that of the neat TPI. Even though they had the same additional contents, the cross-section of the TPI/CNTs@PDA composites had no aggregation, indicating that the PDA further improved the dispersion of the CNTs in the TPI matrix. In addition, with the gradually increased CNTs@PDA content, a large number of small cracks appeared in the composite, and the density and depth gradually increased. This fracture behavior was due to the strong interfacial interaction between the PDA-modified CNTs and the TPI matrix. More effective stress was transferred from the TPI matrix to the CNTs with high modulus, which inhibited the crack propagation in the matrix on a larger scale, leading to the roughness seen in the cross-section of the composites. The addition of the PDA-coated CNTs improved the fracture toughness of the TPI matrix. However, when the content of CNTs@PDA exceeded 2.4 phr, there was still some small-scale aggregation, and the crack density decreased to a certain extent, as shown in Figure 12g. The suggested cause for this could be that with the increased packing content, the CNTs coated with PDA could still have had a strong association with each other, which could have led to their weak dispersion in the TPI matrix.

Shape-Memory Properties
The shape-memory properties of the samples were measured with DMA, and the results are shown in Figure 13 and Table 4. Because the TPI nanocomposites have excellent shape-memory properties, there was little difference in the R f values among the samples, and the R f values of all the samples were maintained at a high level. In addition, the R r of the neat TPI was 99.8%, and the R r of the TPI composite with 0.6 phr of the CNTs and the TPI composite with 0.6 phr of the CNTs@PDA decreased to 98.2% and 97.9%, respectively. The reason for this phenomenon was that the CNTs modified with PDA can effectively promote the crystallization of the TPI matrix. This indicated that the elastic recovery of the TPI decreased as the relative content of the TPI cross-linked networks (at the same density) decreased. Nevertheless, the R r values of all the samples were still greater than 96%, indicating that the TPI composites had excellent shape-recovery properties. These results indicate that the TPI composites had excellent shape-memory performance. dition of the PDA-coated CNTs improved the fracture toughness of the TPI matrix. However, when the content of CNTs@PDA exceeded 2.4 phr, there was still some small-scale aggregation, and the crack density decreased to a certain extent, as shown in Figure 12g. The suggested cause for this could be that with the increased packing content, the CNTs coated with PDA could still have had a strong association with each other, which could have led to their weak dispersion in the TPI matrix.

Shape-Memory Properties
The shape-memory properties of the samples were measured with DMA, and the results are shown in Figure 13 and Table 4. Because the TPI nanocomposites have excellent shape-memory properties, there was little difference in the Rf values among the samples, and the Rf values of all the samples were maintained at a high level. In addition, the Rr of the neat TPI was 99.8%, and the Rr of the TPI composite with 0.6 phr of the CNTs and the TPI composite with 0.6 phr of the CNTs@PDA decreased to 98.2% and 97.9%, respectively. The reason for this phenomenon was that the CNTs modified with PDA can effectively promote the crystallization of the TPI matrix. This indicated that the elastic recovery of the TPI decreased as the relative content of the TPI cross-linked networks (at the same density) decreased. Nevertheless, the Rr values of all the samples were still greater than 96%, indicating that the TPI composites had excellent shape-recovery properties. These results indicate that the TPI composites had excellent shape-memory performance. The reason for this phenomenon was that the CNTs modified with PDA can effectively promote the crystallization of the TPI matrix. This indicated that the elastic recovery of the TPI decreased as the relative content of the TPI cross-linked networks (at the same density) decreased. Nevertheless, the Rr values of all the samples were still greater than 96%, indicating that the TPI composites had excellent shape-recovery properties. These results indicate that the TPI composites had excellent shape-memory performance.
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Conclusions
In this study, we prepared CNTs@PDA nano-fillers via a non-covalent bonding method and then combined it with TPI via a melt-blending method to prepare new shape-memory polymer composites. The effects of different components on the thermal, mechanical, and shape-memory properties of the composites were studied. The SEM results confirm that PDA nanoparticles uniformly coated the surfaces of the CNTs. FTIR, Raman, and XPS spectra were used to determine the chemical structure of the CNTs@PDA. In addition, according to the DSC, XRD, and TGA test results, the crystallization properties and the thermal stability of the samples containing the CNTs@PDA were improved, as compared to those containing only CNTs. Furthermore, the surface and fracture morphologies of the samples were evaluated by SEM, and the results show that PDA could significantly improve the interfacial compatibility of CNTs and TPI, which also improved the mechanical properties of the composites. At the same time, the CNTs@PDA (2.4 phr) composite had the best mechanical properties and shape-memory performance.